1. Field of the Invention
The present invention relates to a mobile communication apparatus and a method of receiving mobile communication signal, and more particularly, is suitably applied to a mobile telephone used as a mobile station of a cellular radio communication system.
2. Description of the Related Art
The cellular radio communication system is so structured that an area where telecommunication service is provided is divided into cells of a desired size and a base station as a fixed station is installed in every cell. A mobile telephone as a mobile station communicates by radio with the base station which is assumed to provide the best communication status.
In general, in the cellular radio communication system, a plurality of mobile telephones share one base station installed in a cell to communicate by radio. This may cause radio wave interference between mobile telephones. In order to avoid such a radio wave interference problem, a variety of systems have been proposed for communication systems between a mobile telephone and a base station. A typical system is the code division multiple access (CDMA) standardized as the Interim Standard-95 (IS-95) system in the U.S.A. and other countries.
The CDMA system is a system in which a different pseudo random noise sequence (PN) code is assigned to each circuit and the PN code is used to spread the bandwidth of transmit signals which have undergone a predetermined modulation over a wider bandwidth (i.e., a spread spectrum modulation), thereby realizing multiplex communication using channels of the same frequency. The receiving side is so structured as to use the same PN code as the transmitting side to apply inverse spread to received transmit signals which have undergone the spread spectrum modulation, thereby demodulating desired transmit signals only.
As described above, in the CDMA system, a different PN code is assigned to every circuit and the transmitting side and the receiving side use the same PN code, so that the received signals are not decoded if the receiving side does not use the same PN code as the transmitting side to perform the inverse spread. The PN code is a pseudo random number series, so that the CDMA system exceeds in privacy.
As shown in FIG. 1, in a cellular radio communication system 1 of the CDMA system, the service area is divided into, for example, a plurality of cells C1 to C7, and base stations BS1 to BS7 is installed in the cells C1 to C7 respectively. The base stations BS1 to BS7 repeatedly transmit the PN codes of the same code pattern to transmit pilot signals at timing different from each other.
A mobile station MS1 checks a correlation value between the pilot signal, which is transmitted from each base station BS1 to BS7 at different timing, and the PN code generated therein. The correlation value represents reception energy of the pilot signal transmitted from the base station BS1 to BS7.
The mobile station MS1 receives a plurality of pilot signals which are transmitted at different timing, and detects such a pilot signal with timing when the reception energy becomes the maximum and thereby, communicates by radio with, for example, the base station BS1 transmitting the pilot signal. Meanwhile, the base station BS1 grasps timing of pilot signals which are transmitted from adjacent base stations BS2 to BS7 as well as the timing of the pilot signal transmitted from its own station, to give the mobile station MS1 notice of a pilot signal with which timing is transmitted from which base station.
In general, the mobile station MS1 changes a base station to be communicated therewith while moving. For example, the mobile station MS1 moves from the cell C1 as the service area of the base station BS1 being currently connected by radio to the cell C2 as the service area of the base station BS2 which is adjacent to the base station BS1.
The mobile station MS1 always monitors the reception energy of pilot signals transmitted from adjacent base stations BS2 to BS7 as well as the reception energy of the pilot signal transmitted from the base station BS1 being currently connected by radio. Accordingly, when the mobile station MS1 moves from the cell C1 to the cell C2, it compares to pilot signals transmitted from other base stations to perform switchover of radio circuits (so called, handoff) to the base station BS2 which transmits the pilot signal of the largest reception energy to the mobile station MS1. In this way, the mobile station MS1 keeps communicating by successively switching the base stations BS even when the mobile station MS1 moves anywhere.
The mobile station MS1 rarely receives radio waves from the base station BS at a certain location which commands a good view, because the mobile station MS1 communicates with the base station BS through a radio circuit. Actually, in most cases, the mobile station MS1 receives composite waves while moving, generated by synthesizing a plurality of reflective waves reflected by obstacles such as buildings and the ground. This changes the reception energy to be detected by the mobile station MS1 every moment as time passes, and may temporarily and considerably attenuate the signal level (so-called, fading).
Any change in the reception energy in the case where the fading occurs will be explained by using FIG. 2. The reception energy E1 shows the measurement results obtained when radio waves from the base station BS1 are received by the mobile station MS1. The reception energy E2 shows the measurement results obtained when radio waves from the base station BS2 are received.
In this case, since the mobile station MS1 locating within the cell C1 is connected by radio to the base station BS1 in the cell C1, it is apparent that the signal level of the reception energy E1 is greater than that of the reception energy E2. However, if the reception energies are detected at the timing of time T1 and T2, the signal levels are reversed due to fading or the like, that is, the signal level of the reception energy E2 is greater than that of the reception energy E1. Accordingly, the mobile station MS1 performs a handoff between the reception energy detection timing of time T1 and the next detection timing and between the reception energy detection timing of time T2 and the next detection timing.
Since the mobile station MS1 has to perform the handoff in a moment, if the fading occurs like that, the mobile station MS performs the handoff even when the handoff is unnecessary, so that it performs an unsuitable operation. Especially, while conversation is in progress, the control data has to be communicated between the mobile station MS1 and the base station BS many times, so that the conversation may be disrupted.
In view of the foregoing, an object of this invention is to provide a mobile communication apparatus and a method of receiving mobile communication signals capable of further preventing an unsuitable operation as compared with the prior art.
The foregoing object and other objects of the invention have been achieved by the provision of a mobile communication apparatus which comprises an antenna for receiving signals transmitted from a plurality of base stations, a demodulating means for demodulating a single received signal out of a plurality of received signals received with the antenna, a detecting means, whenever the detecting means detects each of reception energy values of the plurality of received signals, for repeatedly detecting reception energy values of the plurality of reception signals, and a control means for calculating the average value of reception energy, comparing the average of the reception energy, and demodulating a received signal having the largest reception energy with the demodulating means,
The average value of the reception energy detected this time and the reception energy detected in the past is calculated, and the average value is set as the reception energy, so that further accurate reception energy can be obtained as compared with the case of simply detecting reception energy, even when the detected reception energy temporarily and greatly varies.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.